the big difference is useability.
Surfers like tubes, because they are easy to navigate, and most real tubes dont redirect to other sites. Where mpg's do.
What would you like, one site that you click on whatever video and you stay there to watch the video, or a site that has 50% own content, and the rest is clickthrus
Btw. no money in porn, adapt or die, and tubes dont make money (there, said all the things you might encounter in this thread later on ;))